The immune system must achieve a balance between effective responses to eliminate pathogenic entities and maintaining tolerance to prevent autoimmune disease. T cells are central to preserving this balance, and their proper regulation is primarily coordinated by the B7-CD28 family of molecules. Interactions between B7 family members, which function as ligands, and CD28 family members, which function as receptors, provide critical positive signals that not only initiate, augment and sustain T cell responses, but also contribute key negative signals that limit, terminate and/or attenuate T cell responses when appropriate. A member of the CD28 family, called PD-1 (also known as programmed cell death-1) is upregulated on activated T cells, B cells, and monocytes. PD-1 has two identified ligands in the B7 family, PD-L1 (also known as BH71 or programmed cell death-1 ligand 1) and PD-L2. While PD-L2 expression tends to be more restricted, found primarily on activated antigen-presenting cells (APCs), PD-L1 expression is more widespread, including cells of hematopoietic lineage (including activated T cells, B cells, monocytes, dendritic cells and macrophages) and peripheral nonlymphoid tissues (including heart, skeletal, muscle, placenta, lung, kidney and liver tissues).
More recently, it has been shown that some cancers have developed immune evasion tactics that specifically exploit the PD-1/PD-L1 axis by causing PD-1/PD-L1-mediated T cell exhaustion. Many human tumor cells and tumor-associated antigen presenting cells express high levels of PD-L1, which suggests that the tumors induce T cell exhaustion to evade anti-tumor immune responses. Agents that block the PD-1/PD-L1 pathway are new therapeutic targets for a variety of cancers. To date, PD-1/PD-L1 blockade has been successful in treating a number of cancers, however some patients receiving these treatments develop resistance to these treatments. Thus a need exists for methods to determine which patients are developing resistance to treatment as well as methods for overcoming the resistance.